Method and device for synchronising mobile terminals on a radio channel in direct mode

ABSTRACT

Each communication occupying a direct mode channel comprises the transmission between mobile terminals of signals distributed into traffic time slots and into regularly dispersed control time slots during which one of the mobile terminals transmits synchronization and signaling signals. Some of the terminals determine standby time slots aligned with the control slots and continuing after the communication, and place themselves in a receive state on the channel during the aligned standby slots, with idle periods between said standby slots.

BACKGROUND OF THE INVENTION

Most professional radiocommunication systems offer a functionalitycalled the direct mode or walkie-talkie mode, which allows mobile radioterminals to communicate with one another outside of the coverage of anetwork.

This functionality has several uses: it makes it possible to make up forinsufficient coverage of the network; it ensures continuity of servicein the event of failure or of destruction of the fixed installations ofthe network; finally, it makes it possible to carry out networkindependent operations when the required.

The functions available in direct mode are in general more restrictedthan those available in network mode. One of the functions which is mostoften absent is the terminal standby function.

The standby function consists, for a terminal which is notcommunicating, in placing itself from time to time in a receive state soas to examine the state of the channel and to detect any calls relevantto it. For the remainder of the time, the terminal is sleeping so as tolimit its energy consumption.

In former analog systems, the absence of a standby function did notpresent serious drawbacks. Indeed, the consumption of the reception partof the terminal in the absence of any signal was relatively small.Moreover, most of the terminals were automobile mobile terminals whoseconsumption was not critical.

The digital systems have greater consumption in the reception part, dueto the necessary digital processing. With the development of handheldterminals, this makes terminal standby extremely useful.

In the network mode, the infrastructure controls the instants at whichthe terminals are woken up since it fixes the temporal structure for useof the channels. Synchronization of standby is therefore easilyachieved. However, in direct mode, a form of cooperation must be set upbetween the terminals since the infrastructure may be out of range ornonexistent. Most of the standby mechanisms used in direct modegenerally lead to a loss of the start of the communication or to arelatively long setup time.

European patent 0 786 174 describes a system in which base stations ofthe network supervise the standby function of the terminals both inrespect of the radio resources of the network and in respect of thedirect mode. This approach is suitable only in the particular case wherethe terminals are within range of base stations. This case is not themost frequent.

An object of the present invention is to provide an efficient standbymechanism in direct mode, i.e. one which offers a relatively largeamount of standby and guarantees a relatively short communication setuptime.

It is also desirable for the mechanism to be well adapted to variousmodes of communication setup: mode without acknowledgment (the mostcommon in direct mode), or modes offering a possibility ofacknowledgment, for example by going off-hook after ringing.

SUMMARY OF THE INVENTION

The invention thus proposes a method of synchronizing mobile terminalson a direct mode radio channel, wherein each communication occupying thechannel comprises the transmission between mobile terminals of signalsdistributed into traffic time slots and into regularly dispersed controltime slots during which one of the mobile terminals transmitssynchronization and signaling signals. According to the invention, someat least of the terminals determine standby time slots aligned with thecontrol slots and continuing after the communication, and placethemselves in a receive state on the channel during the aligned standbyslots, with idle periods between said standby slots.

Thus, at the end of the communication, all the terminals which haveparticipated in the communication, or which have synchronized themselveswith the communication without participating therein, can implement thestandby mechanism with mutually synchronized wakeup instants, based onthe synchronization of the previous communication.

This synchronous standby mechanism will thus allow them definitely todetect and to monitor any communication set up on this channel and usingthe same synchronization.

In a preferred embodiment of the method, some at least of the terminalsfurther determine standby time slots which are desynchronized withrespect to the control slots, having a different periodicity from thatof the control slots and continuing after the communication, and alsoplace themselves in the receive state on the channel during thedesynchronized standby slots.

This second offset monitoring by the terminals on standby allows fastdetection of any communication not using the same time setting.

In response to the detection on the channel of signals other thansynchronization and signaling signals in a standby slot, a mobileterminal remains in the receive state on the channel and determines newstandby time slots if it detects synchronization and signaling signals.

This allows the terminal to acquire the synchronization of the directmode channel if it was previously using another synchronization, inparticular if it was disconnected.

Another aspect of the present invention pertains to a control device fora mobile radiocommunication terminal, the terminal comprising means oftransmission and of reception on a direct mode radio channel, wherebyeach communication occupying said channel comprises the transmissionbetween mobile terminals of signals distributed into traffic time slotsand into regularly dispersed control time slots during which one of saidmobile terminals transmits synchronization and signaling signals. Thiscontrol device comprises means of activation of the means of receptionon the channel during standby slots and of deactivation of the means ofreception on the channel outside the standby slots, and means fordetermining time slots, continuing after the communication, which arealigned with control slots positioned in the course of a communicationby detection of synchronization signals transmitted by another terminaland for including said aligned slots in the standby slots.

Another aspect of the present invention pertains to a mobileradiocommunication terminal including such a control device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a radiocommunication system for implementing theinvention;

FIG. 2 is a schematic diagram of a mobile terminal according to theinvention;

FIGS. 3 to 9 are charts illustrating the organization of a direct modechannel into elements of multiframes and the manner of operation ofterminals on this channel.

DESCRIPTION OF PREFERRED EMBODIMENTS

The radiocommunication system represented in FIG. 1 comprises a networkwhose infrastructure comprises one or more base stations 10. These basestations 10 are capable of setting up radio channels with mobileterminals (11-13).

The system considered is a frequency division multiple access (FDMA)system. A certain number of carrier frequencies are devoted to networkmode communications in which the radio transmissions occur between abase station 10 and one or more mobile terminals 11-13.

Another frequency channel is reserved for direct mode communicationsbetween the terminals, as illustrated between the terminals 12 and 13 inFIG. 1. A direct mode communication on the frequency channel may occurwhen one at least of the relevant terminals 13 is outside the coveragearea 15 of the network, but this is not compulsory.

In the exemplary embodiment described below, the direct mode channel isorganized into successive multiframes each comprising a plurality of 360ms elements, each composed of 18 time slots of 20 ms (FIG. 3).

When a communication is set up and activated on the direct mode channel,sixteen of these time slots are devoted to the continuous transmissionof signals representing coded speech or more generally user traffic(traffic slots), whereas the other two time slots serve for the periodictransmission of signals representing synchronization and signalinginformation (control slots), as shown by FIG. 3. In each of FIGS. 3 to9, the time slots drawn with cross-hatching are control slots.

When a dialog is necessary between two terminals, for example for theinitiation of a communication in acknowledged mode, the multiframeelement still comprises two time slots for the periodic transmission ofthe synchronization and signaling information by one or other of theterminals, but the remaining time slots are shared statically betweenthe two directions of communication. This is illustrated by FIG. 4 in aparticular case where the sharing is performed by alternation of blocksof n=3 time slots of transmission by the terminal transmitting thesynchronization and signaling information and of blocks of n=3 timeslots of reception by this terminal, with a vacant time slot with eachchange of direction.

FIG. 2 diagrammatically shows the means with which a terminal 11-13 forcommunicating on the direct mode channel are equipped (the meansrelating to the network mode are not drawn).

A duplexer 20 is linked to the antenna 21 of the terminal so as toseparate the transmission pathway and the reception pathway which arerespectively provided with a radio transmitter 22 and with a radioreceiver 23. A controller 24 ensures the sequencing of the transmitter22 and of the receiver 23 in accordance with the multiframe structureillustrated by FIGS. 3 to 9.

At the output of the radio receiver 23, a demultiplexer 25 distributesthe demodulated signal between a module 26 for the control time slotsand a vocoder 27 for the traffic time slots. Likewise, in thetransmission direction, the controller 24 instructs a multiplexer 28 tosend the output signals from the vocoder 27 to the radio transmitter 22in the traffic time slots, and synchronization and signaling informationemanating from the module 26 in the control time slots if the terminalis the one which ensures the broadcasting of this information.

A time base 29 associated with a crystal oscillator supplies thecontroller 24 with a clock enabling it to obtain the timing of themultiframes and of the 20 ms time slots.

To synchronize the terminal with respect to other terminals engaged in acommunication, the module 26 detects the synchronization and/orsignaling information dispatched in the control time slots, and suppliesa corresponding time marker to the controller 24 which can then instructthe other facilities of the terminal in alignment with the structure ofthe multiframes.

When a direct mode communication is set up and activated, the channel isoccupied by 320 ms sequences of coded speech, intercut with 40 mssequences, used either for the broadcasting of synchronization andchannel state information (characteristics of the communication), or toallow return signaling from the participating terminals to thetransmitter terminal (see FIG. 3).

A terminal which has decided not to participate in the communication,nevertheless has knowledge of the synchronization of the communicationwhich has been detected by its module 26. Its controller 24 can thenimplement a standby mechanism such that it periodically activates theradio receiver 23 so as to monitor part of the information broadcast andthus to ascertain whether the communication is continuing or hasterminated. In the example illustrated by FIG. 5, the terminal wakes upduring a standby slot of 20 ms every 360 ms, aligned with the firstcontrol slot of the multiframe element.

Thus, at the end of the communication, all the terminals which haveparticipated in the communication or which have synchronized with thecommunication without participating therein can continue to apply thesame standby mechanism with mutually synchronized wakeup instants, basedon the synchronization of the previous communication.

This synchronization lasts as long as the drifts of the local time bases29 of each of the terminals do not generate a large offset. Theterminals will then be said to be in a synchronized standby state. Thisstandby mechanism will then allow them definitely to detect and tomonitor any communication set up on the direct mode channel and usingthe same synchronization.

In the event of migration or of power-up of a terminal, its standbycharacteristics are probably not synchronous with those of the terminalswhich have become neighbors. This terminal will be said to be searchingfor synchronization, from the point of view of its mutually synchronizedneighbors.

In this case, if a new communication is set up, the periodic instants ofwakeup of the migrating terminal do not coincide with the instants ofbroadcasting of the synchronization and channel state information.

However, its receiver 23 can, during its wakeup periods, detect thepresence of an energy level above a given threshold, thereby indicatingthat the channel is occupied by signals relating to a communication indirect mode. The controller 24 thus maintains the terminal in thereceive state until a synchronization sequence is detected.

If after decoding of the signaling information accompanying thissynchronization sequence, the terminal determines that the communicationis relevant to it, the controller 24 will activate the receiver 23and/or the transmitter 22 in a corresponding manner.

Otherwise, the terminal places itself in a standby state synchronizedwith the other terminals, in accordance with the mechanism set forthhereinabove. The latter case is that illustrated by FIG. 6.

If the terminal maintained in the receive state after detection ofsignals other than synchronization and signaling signals in a standbyslot does not succeed in subsequently detecting synchronization andsignaling signals, it returns to its standby state preceding the expiryof a timeout whose duration may be of the order of that of themultiframe element.

The latter mechanism operates correctly when the communication occupiesthe entire channel since the continuous transmission by one of theterminals allows definite detection of energy level outside of thecontrol slots.

However, it is not necessarily suitable during the setup phases for acommunication in acknowledged mode. In this case, the initiatingterminal broadcasts synchronization and channel state information with aperiodicity identical to that used for the communications which havebeen set up, but the remainder of the time comprises a dialog (or anattempted dialog) with the terminal to be contacted, and thereforecomprises regular phases of transmission and of reception (or ofawaiting reception), as is illustrated in FIG. 4. The time required forthe acquisition of synchronization by a non-synchronized terminal, i.e.until one of the wakeup instants coincides with a transmission phase,may then become random, or be excessively lengthened.

This drawback is even more serious when the terminal seeking tosynchronize turns out to be precisely the called terminal.

The terminal on standby should then perform a further periodicmonitoring desynchronized with respect to the duration of an element ofthe multiframe, for example by monitoring the channel for an additionalstandby slot of 20 ms every 300 ms.

Thus, whatever case is envisaged, one of the desynchronized standbyslots will quickly coincide with a transmission phase (either periodicsynchronization and channel state information or an element of a setupdialog). The terminal can then trigger the time-delayed search forsynchronization, and thus attain a synchronized standby state, asillustrated by FIG. 7 where the upper part shows the standby slotsaligned with the periodicity of the elements of the control slots andthe lower part shows the desynchronized standby slots.

In the case of a change of speaker, there is also a period during whichtransmission is not continued, and the mechanism illustrated by FIG. 7also improves the speed of synchronization in this transient situation.

It is noted that in order to favor the detection of energy by theterminal when searching for synchronization, the transmitter cancontinue to transmit in all the blocks of n time slots which arereserved for it in the multiframe outside of active communication (seeFIG. 4). In this case, having regard to the static allocation of theblocks reserved for the initiating terminal, the periodicity of thedesynchronized monitorings may be optimized at (360±n×p×20) ms (n and pintegers). Specifically, it is not necessary to scan all the blocks ofthe multiframe.

This mechanism composed of two standby time slots, called alignedstandby slots in respect of those corresponding to synchronous standby(every 360 ms) and desynchronized standby slots in respect of theothers, allows the terminals in all cases:

-   -   to be in a state of synchronized standby with the neighboring        terminals as soon as a communication has taken place; and    -   to quickly acquire the same standby synchronization during        migration or power-up.

The amount of standby offered by this mechanism is compatible with agood energy saving of the battery since the terminal is in the receivestate on average 2 times 20 ms every 360 ms, i.e. around 11% of thetime.

Preferably, during the setting up of a communication in non-acknowledgedmode, the initiating terminal does not begin sending coded speechimmediately, since the terminals which are able to participate in thecommunication and hence to receive the coded speech may be on standby.An initial setup sequence allowing the terminals on standby to detectthe presence of a communication is therefore performed.

Several strategies are possible, according to whether one favors (1) thecertainty that the relevant terminals will be present or (2) as fast aspossible a switch to communication.

In approach (1), the terminal wishing to initiate the communicationbegins to transmit synchronization and signaling information throughoutthe duration separating it from the end of the next aligned standby slotused to wake up all the neighboring terminals, this being the one whichit would itself have used for its next wakeup if the communication hadnot been initiated. As all the terminals wake up at this instant, theirswitch to communication is ensured (FIG. 8). The same holds for most ofthe terminals searching for synchronization, whose standby slots,aligned or desynchronized, will often fall during the initial period oftransmission of the synchronization and signaling information.

In this case, the communication setup time is 220 ms on average and 400ms in the worst case.

It will, however, be noted that the coding of the speech can beginbefore the transmission of the first traffic time slot, thereby reducingthe delay perceived by the user.

In approach (2), at the cost of a small probability of losing the firstfew syllables of the communication, it is possible to further reduce thetime to switch to communication by taking account of the desynchronizedstandby slots. The terminal wishing to initiate the communication innon-acknowledged mode transmits synchronization and signalinginformation for a duration T, equal to the minimum of the duration tremaining until the next aligned standby slot, and of a fixed durationT₀ (FIG. 9).

In this case, all the terminals whose desynchronized standby slot occursin the course of this transmission of duration T are woken up in time,whereas the other terminals will enter the communication after the firsttime slot devoted to the periodic sending of the synchronization andsignaling information of the multiframe, i.e. at the latest (360-T₀) msafter the start of sending of the coded speech (the traffic loss is then(360-T₀) ms).

It is possible to vary T₀ so as to guarantee the best compromise betweenthe average duration of communication setup and the average duration ofcoded speech loss at the start of communication.

During the setup of a communication in acknowledged mode, it isnecessary to ensure the waking up of the terminal to be contacted. Theapproach (1) above should therefore be adopted. If the terminal to becontacted was not synchronized and if none of the standby slots coincidewith the period of initial transmission of the synchronization andsignaling information, the transmitter repeats the setup message atpredetermined instants corresponding to the blocks of n=3 transmissiontime slots of FIG. 4, so that it will certainly be received by theterminal on non-synchronized standby at the latest after three secondarystandby periods, i.e. 900 ms after the start of setup in the embodimentillustrated by FIG. 7.

1. A method of synchronizing mobile terminals on a direct mode radiochannel, wherein each communication occupying the channel comprises thetransmission directly between mobile terminals of signals distributedinto traffic time slots and into regularly dispersed control time slotson said direct mode channel during which one of the mobile terminalstransmits synchronization and signaling signals, the method comprisingthe steps of determining, based on synchronization and signaling signalstransmitted directly between said mobile terminals, by at least some ofthe terminals standby time slots coincided with the control slots onsaid direct mode channel and wherein the standby time slots continue tobe coincided with the control slots after the communication, and placingsaid at least some of the terminals in a receive state on the channelduring the coincided standby slots, with idle periods between saidstandby slots, the method further comprising the following steps inresponse to detection on the channel of signals other thansynchronization and signaling signals in a standby slot by a mobileterminal keeping said mobile terminal in the receive state on thechannel; determining new standby time slots by said mobile terminal ifsynchronization and signaling signals are detected; and determining bysome at least of the terminals standby time slots desynchronized withrespect to the control slots, having a different periodicity from thatof the control slots and wherein the standby time slots continue to becoincided with the control slots after the communication, and placingsome at least of the terminals in the receive state on the channelduring the desynchronized standby slots.
 2. The method as claimed inclaim 1, comprising the step of keeping said mobile terminal in thereceive state on the channel until the expiry of a timeout if nosynchronization and signaling signals are detected.
 3. The method asclaimed in claim 1, further comprising the step of transmittingsynchronization and signaling signals until the next coincided standbyslot from a mobile terminal to initiate a new communication.
 4. Themethod as claimed in claim 1, further comprising the step oftransmitting synchronization and signaling signals from a mobileterminal for a duration equal to a minimum between a predeterminedduration and the duration remaining to run until the end of the nextcoincided standby slot to initiate a new communication innon-acknowledged mode.
 5. A control device for a mobileradiocommunication terminal, the terminal comprising means oftransmission and of reception on a direct mode radio channel, wherebyeach communication occupying said channel comprises transmissiondirectly between mobile terminals of signals distributed into traffictime slots and into regularly dispersed control time slots on saiddirect mode channel during which one of said mobile terminals transmitssynchronization and signaling signals, the control device comprisingmeans of activation of the means of reception on the channel duringstandby slots and of deactivation of the means of reception on thechannel outside the standby slots, and means for determining standbytime slots coincided with control slots on said direct mode channelpositioned in the course of a communication by detection ofsynchronization signals received directly from another terminal and forincluding said coincided slots in the standby slots, wherein the standbytime slots are arranged to continue to be coincided with the controlslots after the communication, wherein the means of activation and ofdeactivation are arranged to activate the means of reception on thechannel until the detection of synchronization and signaling signals inresponse to detection on the channel of signals other thansynchronization and signaling signals in a standby slot, the controldevice further comprising means for determining time slotsdesynchronized with respect to the control slots positioned in thecourse of the communication, having a different periodicity from that ofthe control slots and wherein the standby time slots are arranged tocontinue to be coincided with the control slots after the communicationand means for including said desynchronized time slots in the standbyslots.
 6. The control device as claimed in claim 5, wherein the means ofactivation and of deactivation are arranged to limit the activation ofthe means of reception on the channel to a predetermined timeoutduration in response to detection on the channel of signals other thansynchronization and signaling signals in a standby slot.
 7. The controldevice as claimed in claim 5, further comprising means of initiating anew communication, to instruct the means of transmission on the channelto transmit synchronization and signaling signals until the nextcoincided standby slot.
 8. The control device as claimed in claim 5,further comprising means of initiating a new communication innon-acknowledged mode to instruct the means of transmission on thechannel to transmit synchronization and signaling signals for a durationequal to the minimum between a predetermined duration and the durationremaining to run until the next coincided standby slot.
 9. A mobileradiocommunication terminal, comprising means of transmission and ofreception on a direct mode radio channel and a control device, wherebyeach communication occupying said channel comprises the transmissiondirectly between mobile terminals of signals distributed into traffictime slots and into regularly dispersed control time slots on saiddirect mode channel during which one of said mobile terminals transmitssynchronization and signaling signals, the control device comprisingmeans of activation of the means of reception on the channel duringstandby slots and of deactivation of the means of reception on thechannel outside the standby slots, and means for determining standbytime slots coincided with control slots on said direct mode channelpositioned in the course of a communication by detection ofsynchronization signals received directly from another terminal and forincluding said coincided slots in the standby slots, wherein the standbytime slots are arranged to continue to coincided with the control slotsafter the communication, wherein the means of activation and ofdeactivation are arranged to activate the means of reception on thechannel until the detection of synchronization and signaling signals inresponse to detection on the channel of signals other thansynchronization and signaling signals in a standby slot, wherein thecontrol device further comprises means for determining time slotsdesynchronized with respect to the control slots positioned in thecourse of the communication, having a different periodicity from that ofthe control slots and wherein the standby time slots are arranged tocontinue to be coincided with the control slots after the communicationand means for including said desynchronized time slots in the standbyslots.
 10. The mobile terminal as claimed in claim 9, wherein the meansof activation and of deactivation are arranged to limit the activationof the means of reception on the channel to a predetermined timeoutduration in response to detection on the channel of signals other thansynchronization and signaling signals in a standby slot.
 11. The mobileterminal as claimed in claim 9, wherein the control device furthercomprises means of initiating a new communication, to instruct the meansof transmission on the channel to transmit synchronization and signalingsignals until the next coincided standby slot.
 12. The mobile terminalas claimed in claim 9, wherein the control device further comprisesmeans of initiating a new communication in non-acknowledged mode toinstruct the means of transmission on the channel to transmitsynchronization and signaling signals for a duration equal to theminimum between a predetermined duration and the duration remaining torun until the next coincided standby slot.